During angiographic or other diagnostic procedures which use X-Rays, a system similar to that shown generally in FIG. 1 is used. The system generally includes an X-Ray tube 10 which emits X-Rays from a general point source. The X-Ray tube is positioned under a table 12 on which the patient or other object of interest is positioned. In order to provide for X-Ray photography, cinegraphic recording and/or viewing of the subject, and image intensifier 14 is positioned above the subject.
The bottom surface of the image intensifier 14 includes a grid 16 having a plurality of narrowly spaced strips thereon in order to attenuate scattered X-Rays so that only the X-Rays which pass through the subject directly from the X-Ray tube are recorded by the recording medium 20. The top of the image intensifier includes a TV or similar camera 18 to allow the physician to dynamically view the subject of the study. For example, the heart or other organ of a patient may be viewed through the camera to allow the physician the ensure the proper positioning and observe the operation heart or other organ. Additionally, the top portion of the image intensifier 14 also typically includes a recording medium 20 such as a film camera or digital recording medium to record the study for later review and analysis. The image intensifier incorporates a phosphorous screen 22 and a series of focusing coils 24 which tend to cause the X-Ray beams to be directed toward a phosphorous output lens 26. The beam from the output lens is split by a beam splitter 28 to provide output to the TV camera 18 and recording medium 20.
Current systems which are used to analyze coronary arteries during diagnostic angiographic procedures include automatic analysis programs to calculate the dimensions of the arteries of a patient. While these programs have been used for many years, the programs suffer from certain inaccuracies which result from distortions that occur during the image acquisition process as well as limitations in the sharpness of the pixels of the acquired image. For example, various studies have shown that the accuracy of the analysis programs decreases as the size of the artery of interest decreases and it is the arteries having the reduced sizes that are of particular interest to the cardiologist.
One of the major sources of image distortion with the current systems is geometric distortion. Geometric distortion results in pincushion or barrel distortion of the image and may result from the lenses and focusing coils in the imaging system as well as passage of the X-Rays from the conical or point source of the X-Ray tube to the generally planar grid 16 and image intensifier 14. This type of distortion may result in an image which is concave (pincushion) or convex (barrel) shaped near the edges of the image. Attempts to overcome this type of distortion include calibration of the imaging system when it is installed using a platform phantom having a plurality of lead lines. The lead lines are aligned in a grid shape horizontally and vertically along the platform with a known distance of 1 cm between each other. Although this procedure provides the program with the ability to correct for the calibrated geometric distortion of the system, geometric distortion also arises as the components of the system age or are replaced. Additionally, because there is a strong desire to minimize the dose of X-Rays that the patients are exposed to, the images include noise distortion and the images of the lead lines lose their sharpness around their edges. Additionally, the use of the lead lines does not present an accurate depiction of the absorption of the X-Rays for the organs of interest in a patient because the lead lines distort the X-Rays of the image system in different proportions than the organs of interest of the patient. As a result of the foregoing, the calibration analysis may include a certain amount of error which is then passed on to the calibrated images of the analysis program. Despite these difficulties, it is still desirable to provide an initial or partial correction for geometric distortion.
A further approach to improving image quality and analysis of the artery sizes involves the calibration of the analysis program using the procedure catheter. In the current approach, the outer diameter of the procedure catheter is assigned as a known distance and the areas of interest are then comparatively analyzed based on this distance. Difficulties in this approach arise from the lack of image sharpness inherent in the X-Ray type of imaging system as well as from the many different manufacturers and varieties of catheters which are available today. Further complicating the attempts to calibrate the analysis programs based on the catheter diameter is the fact that the catheters are made of various materials, each of which absorb and scatter the X-Rays differently. Each of these difficulties is then exacerbated by the magnification of the image for use in the analysis program. Despite this, the use of a procedure catheter to calibrate the analysis program is beneficial because the procedure catheter is useful as a reference to compare to the arteries because the absorption characteristics of the X-Rays for the catheters and arteries have greater similarities between each other than the lead lines and arteries. Additionally, the similarity in object size, dimension and object contrast between the procedure catheter and the nearby artery provides a useful reference for identifying the walls of the arteries.
Based on the foregoing, there remains a need for improved calibration or error correction devices and a method of their use to improve the quality of existing analysis programs for imaging systems.
Furthermore, there remains a need for a reliable and consistent calibration or error correction system which may be used to compare the relative differences between imaging systems to allow the images to be analyzed by a common analysis system without introducing additional errors.